Look for signs of your first period. First, your breasts may start to get bigger, hair may start to grow in the pubic area (between your legs) and, a little later, in your armpits. During this time you may notice a whitish discharge from your vagina. This is normal (and it's not urine, which comes from your urethra). A few months after that, you'll probably get your first period. When it actually begins, you might feel dampness on your underwear which is probably a few drops of blood.
From what I've heard from girls and women, you might get cramps, really bad headaches, soreness in your bosom, as well as a wide range of other symptoms. For the most part, you will see blood leaking out of your you know what.